1) Field of the Invention
The present invention relates to a gas concentration detecting apparatus made to detect a concentration of an oxygen concentration and specific gas component of a gas to be detected through the use of a gas concentration sensor for outputting an oxygen concentration signal and a specific gas concentration signal in the basis of the detection results.
2) Description of the Related Art
As such a type of gas concentration detecting apparatus, there has known an apparatus which detects NOx (nitrogen oxides) of an exhaust gas from, for example, a vehicle engine through the use of a limiting-current type gas concentration sensor. The gas concentration sensor has, for example, a three-cell construction including a pump cell, a sensor cell and a monitor cell, and in the pump cell, oxygen of an exhaust gas introduced into a chamber is pumped in and out and the oxygen concentration of the exhaust gas is detected at the same time. Moreover, the sensor cell detects the NOx concentration (concentration of a specific gas component) of the gas after passed through the pump cell, and the monitor cell detects the concentration of the residual oxygen of the in-chamber gas after passed through the pump cell.
The above-mentioned gas concentration sensor can acquire a normal oxygen concentration signal on the condition that the pump cell is in a predetermined active state, and can acquire a normal NOx concentration signal on the condition that the sensor cell is in a predetermined active state. Therefore, in general, the resistance value (for example, element impedance) of a solid electrolyte element forming each of the cells is detected and activation control is implemented through the energizing to a heater (supply of power to a heater) so that the element impedance reaches a target (desired) value corresponding to the active temperature. For example, with respect to the sensor cell, the element impedance is detected, and the energizing to a heater is feedback-controlled on the basis of a deviation between the element impedance detection value and a target value (for example, see Japanese Patent Laid-Open No. 2000-171439).
However, in the case of the existing technique, a decision on the completion of activation of the pump cell and a decision on the completion of activation of the sensor cell are made at the same time regardless of the actual degree of activation of each cell. For this reason, by the time the oxygen concentration signal or the NOx concentration signal meets the requirements for the engine control or the like, there may be a need to wait for approximately several minutes after the start-up of the sensor. This creates a problem in that the requirement for the sensor output to be available in early stages does not reach satisfaction.